Now, to be serious, the reason many people identify Christina one way and Mariah another is the type of (crappy) music they choose to make. Mariah tends to make more R&B sounding music which many people identify with "black" culture while Christina just makes bland, generic pop that is one of the hallmarks of crappy, suburban ie "white" Top 40 stations.I think what people identify as "white" or "black" is more about culture, or perceived culture, than anything else.
If Mariah and Christina where the only two examples, I could very well agree with you, but they aren't. I see these distinctions all the time with every day people. Plus, Mariah didn't always sing R&B and Christina has a very soulful voice. The main reason why I disagree with it being a culture issue is because no one outside the white race is called white and no one outside the black race is called black. We could say that Eminem is culturally black and Colin Powell is culturally white, but they are never referred to as such. It's about skin color, which is why I mentioned the other races being called red, yellow, and brown.